Elucidating gene-environment interactions is crucial for understanding the evolution of behavior. This proposed research evaluates the evolutionary divergence of a mate-recognition system on four scales: molecular, physiological, behavioral, and ecological. I will study Xiphophorus malinche andX. birchmanni, sister species of swordtail fishes, that in individual rivers exist in hybrid gradients. 'Pure' populations of X. malinche are separated from 'pure' populations of X. birchmanni by an upstream-to-downstream gradient of hybrid populations. Along this gradient, I will investigate howvariation in opsin gene sequences, opsin gene expression, and color sensitivity, which is likely to show a strong selection gradient due to the differences in local photic environments, map on to variation in female mate preferences and sexually-dimorphic male traits. Results from the proposed work will provide much needed insight into the ecological and genetic bases of behavioral divergence, and will shed light onto the selective forces and genetic architectures underlying the diversity of animal communicationsystems.